Polystyrene blends and a method of making same

ABSTRACT

Disclosed are blends of polystyrene and at least one of syndiotactic polypropylene, ethylene propylene copolymers, and styrene-butadiene-styrene triblock copolymers. These blends are prepared using solution polymerization and have unique morphologies and desirable physical properties. The blends can also be prepared with graft-promoting or crosslinking agents and rubbers to prepare modified high impact polystyrene.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates polystyrene blends. The present inventionparticularly relates to polystyrene blends prepared using a solutionblend process.

2. Background of the Art

Polystyrene is one of the largest volume thermoplastic resins incommercial production today. Unmodified polystyrene is well suited toapplications where its brittleness is acceptable. Engineering plasticshave been used in applications where less brittleness is required, butsuch polymers are often expensive or have properties other than lessbrittleness that make them less than optimum selections. Thus,styrene-based copolymers, and particularly polystyrene resins that aremodified with organic rubber particles, have been investigated for usein applications requiring less brittleness. The modification ofpolystyrene to reduce brittleness is often referred to increasing itsimpact properties and thus the modified polystyrene is said to havehigher impact.

These high-impact polystyrene blends, commonly referred to by theacronym HIPS, are known to be useful in the art of preparing articleswith polymers wherein the application for the articles requires lessbrittleness than unmodified polystyrene. For example, U.S. DefensivePublication T59,011 to Smith discloses that a high impact resin can beprepared by blending from 15 to 50 parts of an impact modifier with from85 to 50 parts of a clear crystal polystyrene. Such materials aredisclosed to be useful for packaging applications.

Another method of making HIPS is to first dissolve a rubber in styrenemonomer and then polymerize the monomer. Such polymers are disclosed inU.S. Pat. No. 6,569,941 to Sosa, et al. Therein, it is disclosed thatstyrene monomer containing a dissolved polybutadiene rubber is flowedinto an elongated upflow stirred reactor containing three reactionzones, wherein the styrene monomer is polymerized to form a HIPS.

It is also known to blend polystyrene with other materials. For example,U.S. Pat. No. 5,194,525, to Miura et al, discloses a continuous processfor making polystyrene from styrene monomer and a polymerizableunsaturated fatty acid. The inclusion of the unsaturated fatty acid isdisclosed to improve the heat resistance and moldability of the blend,making the modified polystyrene desirable for injection moldingapplications.

In HIPS, desirably the polystyrene is a continuous phase including adiscontinuous phase of rubber particles. The size and distribution ofthe rubber particles in the continuous polystyrene phase can affect theproperties of the HIPS. In blends of polystyrene with other materials,the distribution of the noncontinuous phase in the continuouspolystyrene phase is often similarly important.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a polystyrene blend wherein theblend is a blend of polystyrene and a polymer selected from the groupconsisting of syndiotactic polypropylene, ethylene propylene copolymer,and mixtures thereof, and the blend is prepared by solutionpolymerization.

In another aspect, the present invention is a polystyrene blend whereinthe blend is a blend of polystyrene, rubber, and a polymer selected fromthe group consisting of syndiotactic polypropylene, ethylene propylenecopolymer, hydrogenated styrene butadiene copolymers, and mixturesthereof, and the blend is prepared by solution polymerization.

In still another aspect, the present invention is a process forpreparing a polystyrene blend, the process including admixing styreneand a polymer selected from the group consisting of syndiotacticpolypropylene, ethylene propylene copolymer, and mixtures thereof toform a solution, and then polymerizing the styrene monomer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding and better appreciation of the presentinvention, reference should be made to the following detaileddescription of the invention and the preferred embodiments, taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a photomicrograph made using transmission electron microscopy(TEM) techniques of a polystyrene blend including 5 percent syndiotacticpolystyrene and 6 percent rubber.

FIG. 2 is a TEM photomicrograph of a polystyrene blend including 10percent syndiotactic polystyrene and 6 percent rubber.

FIG. 3 is a TEM photomicrograph of a polystyrene blend including 10percent syndiotactic polystyrene and no rubber.

FIG. 4 is a TEM photomicrograph of a polystyrene blend including 10percent ethylene propylene copolymer and no rubber.

FIG. 5 is a TEM photomicrograph of a polystyrene blend including 5percent isotactic polystyrene and 4% polybutadiene.

FIG. 6 is a TEM photomicrograph of a polystyrene blend including 10percent isotactic polystyrene and 4% polybutadiene.

It will be appreciated that the figures are not necessarily to scale andthe proportions of certain features are exaggerated to show detail.

DETAILED DESCRIPTION OF INVENTION

In one embodiment, the present invention is a blend of polystyrene and apolymer selected from the group consisting of syndiotacticpolypropylene, ethylene propylene copolymer, styrene-butadiene-styrenecopolymers, and mixtures thereof. In another embodiment the blendadditionally includes a rubber.

The polymers useful with the present invention are any that are readilysoluble in styrene at temperatures of from 30° C. to 100° C. Forpurposes of the present invention, readily soluble means soluble at aconcentration of at least 15 weight percent after stirring at about 110°C. to about 1 hour. It is desirable that the polymers be soluble attemperatures that are sufficiently low to avoid thermal polymerizationof polystyrene during the dissolution process.

The syndiotactic polypropylene useful with the present invention is anythat is soluble in styrene at temperatures of from about 30° C. to 100°C. For example, EOD-99-19 from ATOFINA is such a material. In oneembodiment the syndiotactic polypropylene useful with the presentinvention has a melt flow index according to ASTM D-1238 of from about 5to about 50 g/10 minutes. In another embodiment, the syndiotacticpolypropylene useful with the present invention has a melt flow index offrom about 22 to about 27 g/10 minutes. In still another embodiment, thesyndiotactic polypropylene useful with the present invention has a meltflow index of about 25 g/10 minutes. These materials can be preparedusing any catalysts system provided they meet the above solubility andmelt flow properties.

Ethylene propylene copolymers, random or impact, can also be used withthe present invention. Such copolymers can be prepared, for example, byfirst performing a heterocatalyst polymerization of a propylene feedfollowed by a feed of ethylene or a mixture of propylene and ethylene.In the alternative, a compounded form of an ethylene propylene copolymercan be used where an admixture of polyethylene and polypropylene arefirst admixed and then compounded to produce a copolymer. The ethylenepropylene copolymers useful with the present invention can have anethylene content of from about 0.1 to about 14 weight percent or even offrom about 4 to about 12 weight percent. In another embodiment, theethylene propylene copolymers useful with the present invention can havean ethylene content of from about 6 to about 8 weight percent.Commercially available ethylene propylene copolymers useful with themethod of the present invention include, but are not limited toEOD-96-34, FINA 6824 MZ, FINA 4824 WZ, and FINA 7825 from ATOFINA. Anyethylene propylene copolymers that are soluble in styrene attemperatures of from about 30° C. to 100° C. can be used with thepresent invention.

Certain styrene-butadiene-styrene triblock polymers can also be usedwith the present invention. One such triblock polymer is the KRATONG1600 series from KRATON POLYMERS. This material is a linearstyrene-(ethylene-butylene)-styrene triblock. Any such polymer that isreadily soluble in styrene as defined above can be used with the processof the present invention. These materials can be used either with orwithout removal of some or all residual unsaturation.

In the practice of the method of the present invention of preparing apolystyrene blend, a rubber is sometimes included in the components ofthe polystyrene blend. Rubbers useful with the method of the presentinvention include polybutadiene (PB), and styrene-butadiene rubber(SBR). Natural rubbers can also be used. Exemplary rubbers useful withthe present invention include some of the TARKENE® rubbers from BAYERand BUNA® EP rubbers from BAYER. Any rubber that is soluble in styreneat temperatures of from about 30° C. to 100° C. can be used with thepresent invention.

The blends of the present invention are prepared by admixing styrene andsyndiotactic polypropylene, ethylene propylene copolymer, and mixturesthereof. In a first embodiment of the present invention, the polymers ormixture of polymers is soluble, as defined above, in the styrene. In asecond embodiment, a solvent can also be used to increase the solubilityof the polymers in the styrene monomer. Suitable solvents includeethylbenzene, toluene, xylenes, and cyclohexane and mixtures thereof.Any solvent useful to facilitate the full or partial dissolution ofrubber, syndiotactic polystyrene, or ethylene propylene copolymer instyrene monomer that can be removed after polymerization of the styrenemonomer and does not interfere with the polymerization of the styrenemonomer can be used with the method of the present invention. In thisembodiment of the present invention, the breadth of polymers and rubbersuseful with the present invention is increased to include those polymersand rubbers that are soluble as defined above in the solvent and styreneadmixture.

In the practice of the process of the present invention, the admixtureof monomer and polymer or polymer mixture is further admixed with apolymerization catalyst. This process is a solution polymerizationprocess. Exemplary catalysts include peroxide catalysts such as, but notlimited to Lupersol® 331 (1,1-di-(t-butylperoxy)cyclohexane Lupersol®531 (1,1-di-(t-amylperoxy)cyclohexane); Lupersol 231(1,1-di-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane); Lupersol TAEC(OO-t-amyl-O-(2-ethylbexyl monoperoxy-carbonate); Lupersol TBIC(OO-t-butyl O-isopropyl monoperoxy-carbonate); Lupersol TBEC(OO-t-butyl-O-(2-ethylhexyl)monoperoxy-carbonate); Trigonox® 17(N-butyl-4,4-di(t-butylperoxy)valerate); and Lupersol 233 (Ethyl3,3-Di-(t-butylperoxy)butyrate). Other catalysts that can be used withthe method of the present invention any peroxide having a one-hourhalf-life of form 80° C. to 160°.

The polymerization of the styrene monomer can be done using any methodknown to be useful to those of ordinary skill in the art of preparingpolystyrene blends. For example, the polystyrene blends can be preparedusing an upflow reactor. The polymerization process can be eithercontinuous or done in batches. The temperature ranges useful withprocess of the present invention can be selected to be consistent withthe operational characteristics of the equipment used to perform thepolymerization. In one embodiment, the temperature range for thepolymerization can be from about 100° C. to about 230° C. In anotherembodiment, the temperature range for the polymerization can be fromabout 120° C. to about 150° C.

The blends of the present invention can also be prepared using, asadditional components, graft promoters, such as maleic anhydride; chaintransfer agents, such as dodecyl mercaptan; or a crosslinking agent,such as divinyl benzene. Exemplary graft promoting, chain transfer, andcrosslinking agents include dodecyl mercaptan, maleic anhydride anddivinyl benzene, and mixtures thereof. The use of such crosslinkingagents with the process of the present invention can increase thecompatibility of the components and further improve the properties ofthe blends made therewith. When used, the graft-promoting crosslinkingagent can be used at levels of from about 100 to about 10,000 ppm. Inone embodiment of the present invention, the graft-promotingcrosslinking agent is used at levels of from about 1000 to about 8000ppm. The use of a crosslinking agent can be useful in polystyrene blendsof the present invention wherein the blend does not contain a rubber.

One embodiment of the present invention is a blend of polystyrene withsyndiotactic polypropylene. In styrene blends where the blend iscomposed primarily of polystyrene and syndiotactic polypropylene, theweight ratio of polystyrene to syndiotactic polypropylene, PS:sPP, canbe from about 19:1 to about 4:1. In another embodiment, the the weightratio of polystyrene to syndiotactic polypropylene, PS:sPP, can be fromabout 15:1 to about 10:1.

In another embodiment, the present invention is a blend of polystyrenewith ethylene propylene copolymer. In styrene blends where the blend iscomposed primarily of polystyrene and ethylene polypropylene copolymer,the weight ratio of polystyrene to ethylene polypropylene copolymer,PS:EPC, can be from about 19:1 to about 4:1. In another embodiment, theweight ratio of polystyrene to ethylene polypropylene copolymer, PS:EPC,can be from about 15:1 to about 10:1.

In still another embodiment, the present invention is a blend of a highimpact polystyrene, that is a mixture of rubber and styrene, withsyndiotactic polypropylene. In styrene blends where the blend iscomposed primarily of HIP and syndiotactic polypropylene, the weightratio of HIP to syndiotactic polypropylene, HIP:sPP, can be from about19:1 to about 4:1. In another embodiment, the the weight ratio ofpolystyrene to syndiotactic polypropylene, PS:sPP, can be from about15:1 to about 10:1.

In another embodiment, the present invention is a blend of polystyrenewith HIP. In styrene blends where the blend is composed primarily of HIPand ethylene polypropylene copolymer, the weight ratio of HIP toethylene polypropylene copolymer, HIP:EP, can be from about 19:1 toabout 4:1. In another embodiment, the weight ratio of polystyrene toethylene polypropylene copolymer, PS:EP, can be from about 15:1 to about10:1.

When the polystyrene blend is a HIP blend, the weight ratio ofpolystyrene to rubber, PS:RUBBER, is from about 99:1 to about 7:1. Inanother embodiment, the weight ratio of polystyrene to rubber,PS:RUBBER, can be from about 19:1 to about 10:1. Included in the term“rubber” are any of the materials already described above.

One advantage of a blend over unmodified polystyrene is that it can havebetter impact properties and can also retain blowing agents, such ascarbon dioxide in foam applications. Another application of thecompositions of the present invention is in the use of containers incontact with food items such as salad oil. The blends of the presentinvention also compare favorably to conventional HIPS. The blends of thepresent invention can have morphologies that have better dispersion ofthe non-continuous phase within the continuous polystyrene phase.Preferably, the non-continuous components are evenly distributed,intermixed and exist as spheres rather than as elongated cylinderswithin the continuous polystyrene phase of the blends. This effect canbe enhanced using graft-promoting agents, such as maleic anhydride. Thenet effect of using polyolefins is to modify the impact and solventproperties of the polystyrene composites. Increased ductility isobtained by enhancing the rubber phase volume of the composite.

The polystyrene blends of the present invention can be prepared usingadditives. Exemplary additives include fillers such as talc,anti-oxidants, UV stabilizers, mineral oil, and the like. Any additiveknown to be useful in preparing polystyrene blends to those of ordinaryskill in the art of preparing such blends can be used with the presentinvention.

In an embodiment of the present invention wherein there is residualmonomer at the end of the polymerization of the styrene monomer, themonomer can be removed from the polystyrene blend. In embodiments wherea solvent is used, the solvent can be removed from the polystyreneblend. Any method of removing unreacted styrene monomer and solvent, ifany, known to be useful can to those of ordinary skill in the art ofmanufacturing polymer blends can be used with the method of the presentinvention. After removal, the solvent and styrene monomer can berecycled or discarded.

EXAMPLES

The following examples are provided to illustrate the present invention.The examples are not intended to limit the scope of the presentinvention and they should not be so interpreted. Amounts are in weightparts or weight percentages unless otherwise indicated.

Example 1

A HIP polystyrene blend is prepared by dissolving 5 percent of asyndiotactic polypropylene sold under the trade designation EOD 99-19 byATOFINA and 6 percent of a percent rubber sold under the tradedesignation TAKTENE® 550 T by BAYER in styrene monomer at 110° C. 300ppm Lupersol® L-531, 100 ppm Lupersol L-233 and 400 ppm maleic anhydrideare added and the solution held at 110° C. for 120 minutes. Theadmixture temperature is raised to 130° C. and held for 60 minutes. Theadmixture temperature is raised to 150° C. and held for 60 minutes.During the course of and at the end of the heating periods, samples areremoved and devolatized at 225° C. for 25 minutes at a pressure of 0.8torr (107 Pascal). The solids level in the samples is about 70%. Asample of the final polymer is subjected to transmission electronmicroscopy to prepare a photomicrograph that is shown below as FIG. 1.It takes about 5–10 minutes for dissolution of the syndiotacticpolypropylene in the styrene monomer.

Example 2

Example 1 is repeated and tested substantially identically except that10 percent syndiotactic polypropylene is used and the TEMphotomicrograph is shown below as FIG. 2.

An examination of the electron photomicrographs of the polymers ofExample 1 and Example 2 (FIGS. 1 & 2) shows that the polymers (101 &201) have a continuous polystyrene phase (102 & 202) and, distributedtherein, inclusions of syndiotactic polystyrene and polybutadiene. Theseinclusions consist of primarily two types. A first type (103 & 203)consists of a honeycomb type structure of black polybutadiene andsyndiotactic polystyrene. A second structure (104 & 204) consists of asphere of syndiotactic polystyrene surrounded by a layer or membrane ofblack polybutadiene.

Examples 3–6

The procedure of Example 1 is repeated substantially identically exceptthat components are varied as shown below in Table 1. The samples, priorto being devolatilizing, are analyzed for physical properties and theresults as displayed below in Table 1.

TABLE 1 Formulations and Results of HIPS Prepared in the Presence ofSyndiotactic Polypropylene And Ethylene Propylene Copolymer And MaleicAnhydride Example 3 Example 4 Example 5 Example 6 Feed % Syndiotactic 510 Polypropylene¹ % Ethylene 5 10 Propylene Copolymer² % Rubber³ 8 8 8 8Maleic 1200 1200 400 400 Anhydride (PPM) Lupersol 531 300 300 300 300(PPM) Lupersol 233 100 100 100 100 (PPM) Properties RPS microns⁴ 9.0 7.92.8 2.4 % rubber⁵ 16.5 15.4 6.6 6.4 % sPP or EP⁶ 10.4 19.3 6.1 12.5 %gels⁷ 43.8 47.4 18.9 22.4 Swell Index⁸ 7.7 7.9 11.7 8.2 Gel/rubber⁹ 2.73.1 2.9 3.5 DMA¹⁰ rubber Tg ° C. −85.4, 2.0 −86.6 −87.1, −14.4 Tan delta0.11, 0.053 0.049 0.041, 0.028 EOD 99-19 is a syndiotactic polypropylenehaving a MFI of 12 available from ATOFINA. EOD 94-21 is an ethylenepropylene copolymer having a MFI of 12–35available from ATOFINA.TAKTENE 550 T is a polybutylene rubber having a Mooney Viscosity of48–55 available from BAYER.4–10. Test methods. RPS, rubber particle size, is obtained via MalvernParticle Size Analyzer in methyl ethyl ketone; % rubber is obtained bytitration with iodine monochloride; % sPP or EP in the product isobtained by calculation as follows: a) % sPP or EP in thefeed/1.0—unreacted styrene; b) swell index is obtained by dissolvingsamples in toluene, separating the gel phase by centrifugation, andobtaining the ratio of the wet gel to dry gel; c) gel/rubber ratio isobtained by calculation from parameters shown within the table, and d)Tg and tan delta values are obtained by using a Rheometrics® RDA IIdynamic mechanical analyzer.

Examples 7–12

The procedure of Example 1 is repeated substantially identically exceptthat components are varied as shown below in Table 1. The samples, priorto being devolatilizing, are analyzed for physical properties and theresults as 15 displayed below in Table 2.

TABLE 2 Formulation and Results of HIPS Syntheses in the Presence ofSyndiotactic Polypropylene, sPP, and Ethylene Propylene Copolymer, EP,and Maleic Anhydride, MA. Example 7 8 9 10 11 12 Formulation % sPP¹ 1212 12 % EP² 12 12 12 % Rubber³ 4 4 4 4 4 4 Lupersol L-531 300 300 300300 300 300 PPM Lupersol L-233 100 100 100 100 100 100 PPM MA PPM 20004000 8000 2000 4000 8000 Pellet Properties % Rubber 6.5 6.3 7.0 6.3 6.37.4 RPS⁴ 5.7 6.9 6.6 2.5 2.8 3.4 % Gel⁵ 36.5 39.2 38.4 27.5 34.0 38.7Swell lndex⁶ 5.2 4.9 5.3 5.9 4.7 4.2 Gel/Rubber⁷ 5.6 5.77 5.5 4.4 5.45.2 Gel/ 2.0 2.1 2.0 1.5 1.9 2.0 (% PB + PO)⁸ Mn (000)⁹ 112 103 106 88105 84 Mw (000)¹⁰ 258 235 248 231 247 210 MWD¹¹ 2.3 2.3 2.3 2.6 2.4 2.5^(1–11)See Table 1

Example 13

A polystyrene blend is prepared by dissolving 10 percent syndiotacticpolypropylene, sold under the trade designation EOD-94-21 by ATOFINA, instyrene monomer at 110° C. 300 ppm Lupersol L-531, 100 ppm LupersolL-233 and 8000 ppm maleic anhydride are added and the solution held at110° C. for 120 minutes. The admixture temperature is raised to 130° C.and held for 60 minutes. The admixture temperature is raised to 150° C.and held for 60 minutes. During the course of and at the end of theheating periods, samples are removed and devolatized at 225° C. for 25minutes at a pressure of 0.8 torr (107 Pascal). The solids level in thesamples is about 70%. A sample of the final polymer is subjected totransmission electron microscopy to prepare a photomicrograph that isshown below as FIG. 3.

The morphology of the photomicrograph of Example 13 (FIG. 3) shows thatin a polystyrene blend with syndiotactic polypropylene (301), thesyndiotactic distributes as small spheres (303) having entrappedpolystyrene (304) in a continuous polystyrene phase (302). Note that norubber is used.

Example 14

A polystyrene blend is prepared by dissolving 10 percent ethylenepropylene copolymer [details . . . either product name or else propertylisting] in styrene monomer at 110° C. 300 ppm Lupersol L-531, 100 ppmLupersol L-233 and 8000 ppm maleic anhydride are added and the solutionheld at 110° C. for 120 minutes. The admixture temperature is raised to130° C. and held for 60 minutes. The admixture temperature is raised to150° C. and held for 60 minutes. During the course of and at the end ofthe heating periods, samples are removed and devolatized at 225° C. for25 minutes at a pressure of 0.8 torr (107 Pascal). The solids level inthe samples is about 70%. A sample of the final polymer is subjected totransmission electron microscopy to prepare a photomicrograph that isshown below as FIG. 4.

The morphology of the photomicrograph of Example 14 (FIG. 4) shows thatin a blend of polystyrene and ethylene propylene copolymer (401) theethylene propylene copolymer distributes as larger spheres, generallynot having entrapped polystyrene (403), in a continuous polystyrenephase (402).

Comparative Examples I & II

Examples 1 and 2 are repeated substantially identically except that anisotactic polypropylene, sold under the trade designation FINA 4621 isused, is used. Samples of the final polymers are subjected totransmission electron microscopy to prepare a photomicrograph that isshown below as FIG. 5 for Comparative Example I which has 5 percentisotactic polypropylene, and FIG. 6 for Comparative Example II which has10 percent isotactic polypropylene. It takes about 20–30 minutes todissolve the isotactic polypropylene in the styrene monomer.

An examination of the photomicrographs of the polymers of ComparativeExamples I & II (FIGS. 5 & 6) shows that in a styrene blend of isotacticpolypropylene (501 & 601), the isotactic polypropylene form irregularpools (503 & 603) in a continuous polystyrene phase (502 & 602).

1. A polystyrene blend comprising a blend of polystyrene and a polymerselected from the group consisting of syndiotactic polypropylene,ethylene propylene copolymer, and mixtures thereof, wherein the blend isprepared by solution polymerization and the polystyrene blend comprisesfrom about 80 wt. % to about 95 wt. % polystyrene.
 2. The polystyreneblend of claim 1 additionally comprising performing the solutionpolymerization using a graft-promoting agent.
 3. The polystyrene blendof claim 2 wherein the graft-promoting agent is maleic anhydride.
 4. Thepolystyrene blend of claim 1 additionally comprising performing thesolution polymerization using a chain transfer agent.
 5. The polystyreneblend of claim 4 wherein the chain transfer agent is dodecyl mercaptan.6. The polystyrene blend of claim 1 additionally comprising performingthe solution polymerization using a cross linking agent.
 7. Thepolystyrene blend of claim 6 wherein the cross linking agent is divinylbenzene.
 8. The polystyrene blend of claim 1 additionally comprising anadditive.
 9. The polystyrene blend of claim 8 wherein the additive isselected from the group consisting of fillers, anti-oxidants, UVstabilizers, mineral oil, and mixtures thereof.
 10. A polystyrene blendcomprising a blend of polystyrene and syndiotactic polypropylene whereinthe blend is prepared by solution polymerization and the weight ratio ofpolystyrene to syndiotactic polypropylene, PS:sPP, is from about 19:1 toabout 4:1.
 11. The polystyrene blend of claim 1 wherein the blend iscomposed primarily of polystyrene and ethylene polypropylene copolymer,and the weight ratio of polystyrene to ethylene polypropylene copolymer,PS:EPC, is from about 19:1 to about 4:1.